Pillion seat issue

I'll apologise for asking as it's been cover ed before and I have searched for answers,
I would like to take my daughter out on bike but..... the pillion seat I bought doesn't seem to fit, it came from a 99 rsv , so I was told, but when I put it on the bike it won't lock in. The catch at the back is fine but where the plastic hooks under the frame it's quite far away meaning the seat can be lifted with ease and it doesnt look right, seat looks too big, has anyone had this problem , I think I was sent the wrong seat but im told it's the right one, any help in getting a nagging teenage daughter off my back would be great